Three primate lentiviruses (simian immunodeficiency viruses, SIV) have been isolated from a macaque (M. nemestrina) housed at the University of Washington Primate Center (SIV/Mne), from a wild-caught mangabey (SIV/Cat) and from a colony-housed African green monkey (SIV/Cae). The structural proteins of these viruses have been purified, the amino-terminal amino acid sequences determined, and polyclonal antisera produced. SIV/Mne has been inoculated intravenously into three species of macaques; virus was isolated from all animals who subsequently have died at 15 to 140 weeks (mean 84 weeks) with CD4 cell depletion and other manifestations of immune deficiency. A single-cell clone (clone E11S) has been obtained from infected HuT-78 cells that produces virus particles containing large amounts of gpl2O envelope protein associated with the virus, even after sucrose gradient purification. A molecular clone obtained from clone EllS has been completely sequenced and shown to be 82% identical to human immunodeficiency virus-2 (HIV-2): This full-length clone is infectious and pathogenic; both macaques infected with the molecular clone have exhibited a marked decrease in CD4+ peripheral blood lymphocytes, and one animal died at 83 weeks. The only change detected in the molecular clone after one year of replication in infected macaques was the loss of the transmembrane protein stop codon. Single-cell clones of an Epstein-Barr virus-transformed B cell line have been obtained that readily form giant cells (syncytia) after infection with SIVs isolated from macaques, mangabeys, African green monkeys, HIV-2, and HIV-1. These clones have been used in the development of a microtiter plate assay to measure the inhibition of virus growth by primate and human serum samples. This assay has permitted the identification of several peptides in the envelope region of SIV that elicit neutralizing antibodies after immunization of macaques.